Photomultiplier tube with a low energy electron inhibitor electrode



Nov. 5, 1968 E. H. EBERHARDT 3,409,778

PHOTOMULTIPLIER TUBE WITH A LOW ENERGY ELECTRON INHIBITOR ELECTRODE Filed May 25, 1966 I I I IIIML INVENTOR. Hward/zfbara d t BY WW1 A44 United States Patent 3,409,778 PHOTOMULTIPLIER TUBE WITH A LOW ENERGY ELECTRON INHIBITOR ELECTRODE Edward H. Eberhardt, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation, Nutley,

N.J., a corporation of Maryland Filed May 25, 1966, Ser. No. 552,732 7 Claims. (Cl. 250-207) The present invention relates to an improved phototube and more particularly to a phototube providing a significantly improved signal-to-noise ratio.

The present invention relates to a phototube of the type disclosed and claimed in Nevin US. Patent No. 2,796,547. In this prior device, it was found that by providing an electron-defining aperture in the image plane of focused electrons emitted from a photoelectric cathode that certain noise caused by thermal electron emission could be minimized to an extent as to provide a significant increase in signal-to-noise ratio. The present invention constitutes a decided improvement thereover in the respect of further discriminating against spurious electron emission such that even a greater improvement in signal-to-noise ratio may be realized. This is accomplished primarily by discriminating against spurious emission thereby, in effect, reducing noise generation.

It is therefore an object of this invention to provide a phototube of the same type in which spurious electron which the signal-to-noise ratio is increased.

It is another object of this invention to provide a photo-tube of the same type in which spurious electron emission is discriminated against or otherwise rejected thereby reducing the quantity of so-called noise emis- In accordance with the present invention, there is provided a phototube which includes a photoelectric cathode, a masking electrode having an electron-receiving aperture which is spaced a predetermined distance from the cathode, an electron lens interposed between the cathode and the masking electrode for accelerating and focusing the electrons emitted by the cathode into an electron image plane which includes the aforesaid aperture, these electrons being accelerated to a relatively high energy level by the electron lens before passing through the aperture, means for collecting the high energy electrons which pass through the aperture, and means inhibiting the passage of lower energy electrons through the aperture, these latter electrons being the products of spurious emission which produces unwanted noise. More specifically, the photoelectric cathode is of extended area and the electron lens includes a first conducting sleeve conductively connected at one end to the cathode and extending toward the masking electrode. The lens additionally, includes a second conductive sleeve smaller than the firstw'hich is supported coaxially with respect thereto, this second sleeve being closed by the masking electrode at the end remote from the cathode with the exception of the aforesaid aperture. The opposite end portion of the second sleeve is closed with the exception of a centrally located opening, thereby providing a substantially field-free space internally of the second sleeve, the latter being substantially closed with the exception of the aforesaid aperture and opening. The electron-inhibiting means includes an electrode located between the masking aperture and the collecting means, this electrode being normally biased somewhat negatively with respect to the masking electrode so as to present a potential barrier to low energy electrons to prevent them from leaving the field-free space via the masking aperture. The biasing potential, however, is not great enough to prevent the signal electrons from penetrating the aperture and reaching the electron-collecting means.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is an axial section of one embodiment of this invention showing the manner of connecting operating potentials thereto; and

FIG. 2 is a diagrammatic illustration thereof which is used in explaining the phototube operation.

Referring to the drawings, an evacuated, cylindrical envelope 1 is provided on its left-hand end wit-h a conventional, transparent faceplate 2 and on its right-hand end with a base 3 for supporting suitable terminal pins 4. A photoelectric material, indicated by the numeral 5, is coated on the inner face of the end plate 2, this material including any of the well-known photoelectric compositions such as silver-oxide-cesium. This cathode arrangement is conventional and serves to emit electrons in respouse to radiant energy focused on the left-hand surface thereof through the transparent faceplate 2.

An electron lens indicated generally by the reference numeral 6 is supported within the envelope 1 in cooperative relation with the cathode 5. This lens comprises a cathode sleeve 7, cylindrical in shape, conductively connected at its left-hand end to the cathode 2 and an accelerating or anode sleeve 8 which is smaller and coaxially extends a short distance into the sleeve 7. An apertured member 9 is fitted into the left end of the cylinderical sleeve 8 and has a small opening 9a serving a purpose which will be explained later. A masking disc or electrode 10 having a central beam-defining aperture 11 is conductively connected to the right-hand end of the sleeve 8. The sizes of the sleeves 7 and 8 and the positions thereof are somewhat critical and can be determined by conventional experimental or mathematical techniques to comply with given design requirements. For the purposes of this invention, it is only necessary to state that the structure of the electron lens 6 is such as will focus an electron image emitted by the cathode 5 onto an image plane coincident with the masking electrode 10 and aperture 11. The method and structure for obtaining an electron image in this so-called image plane are well known and need not be further elaborated here.

The sleeve 8 and the two end elements 9 and 10 provide an essentially closed tube element with the exception of the two openings 9a and 11. Thus, there is little, if any, electric field inside the sleeve 8 which serves a purpose to be explained more fully later on. Additionally, the sleeve 8 is supported inside the envelope 1 by means of a solid conductive disc 8a which, in conjunction with the sleeve structure itself, substantially isolates that portion of the phototube on the left side of the disc 8a from that portion on the right side with the exception of the two openings 9a and 11. The reason for this will become apparent from the later description.

A multi-stage electron multiplier, indicated by the reference numeral 12, is mounted adjacent to the masking electrode 10 opposite the aperture 11. Electrons passing through the aperture 11 will therefore impinge the first stage 12a of the multiplier which emits secondary electrons at a rate greater than unity, these electrons being multiplied by succeeding stages in a manner well known to the art. This multiplier may be conventional and serves the purpose of increasing the magnitude of the electronsignal passed through the masking aperture 11.

Interposed between the masking electrode 10 in the first stage 12a of the multiplier 12 is an electron-inhibiting or control electrode 13 in the form of a fiat metal disc having an aperture 14 slightly larger than the aperture 11 coaxially aligned between the first multiplier stage 12a and the aperture 11. Thus, an electron beam of sufficiently high energy that passes between the aperture 11 and the first stage 12a will also pass through opening 14. The disc 13 is fixedly supported in the envelope 1 by suitable insulating means in this position so that a biasing potential may be applied thereto separate and apart from sleeve 8 and multiplier 12.

Operating potentials may be connected to the various electrodes just described in such a manner as to apply ground potential to cathode 5 and sleeve 7 and a potential which is positive with respect thereto to the anode sleeve 8 (masking electrode 10 and end member 9 being conductive plate-like elements connected to sleeve 8). A positive potential is connected to the multiplier 12 and successively higher potentials are applied to the succeeding stages in accordance with conventional practice. Ad-

ditionally, a potential negative with respect to that applied to the anode sleeve 8 is applied to the control electrode 13.

In operation, an optical image projected onto the lefthand surface of the faceplate 2 will serve to excite cathode 5. Electrons will be emitted from that much of the surface of the cathode which receives the optical image, and these electrons are accelerated to relatively high energies and focused through opening 9a and into the plane of aperture 11. Only those electrons which fall within the area of aperture 11 will be utilized, since they pass through onto the first stage of multiplier 12 after also passing through the control aperture 14. The bias voltage applied to the control electrode 13 is adjusted such that it will not interfere with the electron-multiplying energies of these electrons. The sizes of the electrodes, and more particularly the diameter of the cathode 5 and the sizes of the apertures 9a and 11, are so selected that only the useful, image-receiving portion of the cathode 5 is effective in producing the electrons which are accelerated and focused through the aperture 11. All other electrons emitted from the cathode area radially outside this elfective area will not be useful in generating wanted signal, and these will be precluded from passing through the aperture .1 1 by the presence of the masking electrode 10 itself as well as the walls of the anode sleeve 8.

Restating the same, an optical image may be cast on only that portion of cathode 5 which is in registry with the masking aperture so that the remaining emission by parts of the cathode outside the effective area as well as by other tube parts will be focused not into the aperture 11 but against the masking electrode -10 or some other part of the sleeve 8. This will be explained in more detail later on. However, since this emission is intercepted by the masking electrode 10, it can generate but only negligible noise, if any.

Referring more particularly to FIG. 2, bracket 15 is used to denote the effective area of the cathode 5 for purposes of further explaining the operation of this invention. The solid-like arrows leading from the cathode 5 through the various openings in the electrode to the first stage 12a of the multiplier are representative of the paths followed by the cathode electrons emitted from the efiective area 15 which are focused into the plane of and through the defining aperture 11. The dashed arrow 16 is used to denote the path of electrons emitted from the portion of the cathode and sleeve 7 structure which, while focused or directed through the opening 9a, strike the wall of the sleeve 8. If this peripheral emission 16 is of high enough velocity, secondary electrons will be emitted from the point of impact on the anode sleeve 8. Since the anode sleeve 8 is substantially closed and the space there-inside therefore is correspondingly field-free, these secondarily emitted electrons will have little or no force exerted thereon so will tend to drift randomly. Some field, however, may extend through the aperture 11 from the multiplier stage 12a such that 1, 56 secon ry electrons will tend to drift toward the aperture '11; however, by biasing the electrode 13 sufiiciently negative with respect to the masking electrode 10, a repelling field will be presented to the space within the aperture 11 which rejects or otherwise repels the secondary and other spurious electrons away from the aperture 11. Thus, spurious noise electrons are prevented from passing through the aperture 11 and from reaching the first stage 12a of the multiplier.

Spurious electron emission occurring for any of a variety of reasons may be discriminated against at the aperture .1'1 and substantially eliminated from the useful beam which eventually reaches the first stage 12a. Spurious electrons resulting from thermal emission, X-ray bornbardment, ionization and the like all can be discriminated against in this same manner. Disc 8a also serves in preventing low energy spurious electrons from passing from the left side thereof into the multiplier section. Only that spurious emission which is emitted within the useful crosssectional area 15 of the cathode *5 and which is acted upon by the accelerating field of the electron lens can ever penetrate the defining aperture 11. It is therefore seen that by reducing the spurious electron emission that ultimately reaches the multiplier 12, the resultant signalto-noise ratio is significantly improved.

In a working embodiment of this invention, and referring more particularly to FIG. 1, the following dimensions and operating voltages may be used. It should be understood, however, that these dimensions and voltages as limitations:

Diameter of cathode 5 inch 0.8 Diameter of sleeve 7 do 0.8 Length of sleeve 7 do 1.0.. Diameter of sleeve 8 do 0.5 Length of sleeve 8 do 0.9

Distance between opening 9a and aperture 11 inch 0.9

Distance from right-hand end of sleeve 7 and location of aperture 9a inch 0.1 Size of opening 911 do 0.15 Size of aperture 11 do 0.070 Size of aperture 14 do 0.1

Distance between masking electrode 10 and disc 13 inch-.. 0.010 Distance between disc 13 and stage 12a do 0.030

Voltage applied to cathode 5 and sleeve 8 (variable) v. DC..- 200-800.

Voltage applied between electrode 13 and sleeve 8 (variable) v 5-20 While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

1. A phototube comprising a photoelectric cathode, a masking electrode provided with an electron-receiving aperture and spaced a predetermined distance from said cathode, electron lens means interposed between said cathode and said masking electrode for accelerating and focusing the electrons emitted by said cathode in an electron image plane which includes said aperture, said electrons being accelerated to a relatively high energy level by said lens means before passing through said aperture, means for collecting the high energy electrons which pass through said aperture, and means inhibiting lower energy electrons from passing through said aperture thereby preventing at least a portion of such lower energy elec- 3. The phototube of claim 2 in which said electric fieldcreating means includes a control electrode having an opening coaxially arranged between said aperture and said collecting means, and a source of potential connected between said masking electrode and said control electrode which renders the latter negative with respect to said masking electrode.

4. The phototube of claim 2 wherein said electric fieldcreating means includes an electron-repelling electrode having an opening coaxially arranged between said aperture and said collecting means, said electron-repelling electrode having terminal means for applying an electron-repelling potential thereto.

5. The phototube of claim 4 in which said cathode is of extended area, said electron lens including a first conducting sleeve conductively connected at one end to said cathode and extending toward said masking electrode, and a second conductive sleeve smaller than the first and being supported coaxially with respect thereto, said second sleeve being closed except for said aperture at the end remote from said cathode by said masking electrode, the opposite end portion of said second sleeve being closed with the exception of a centrally located opening thereby providing a substantially field-free space internally of said second sleeve; said electron-repelling electrode being conductive and extending radially outwardly beyond the opening therein, and a source of biasing potential connected to said terminal means for confining low energy electrons within said field-free space and thereby control penetration of these low energy electrons through said masking aperture.

6. A phototube comprising a photoelectric cathode,

-and means for retarding the flow of electrons through said aperture to said collecting means whereby lower energy electrons within said field-free space will be prevented from penetrating said aperture.

7. The phototube of claim 6 wherein said last-mentioned means and said retarding means include conductive elements, respectively, and means for applying potentials to said elements and to said collecting means which renders the element of said retarding means more negative than the element of said last-mentioned means and said collecting means.

References Cited UNITED STATES PATENTS 2,796,542 6/1957 Navin 313 X 2,895,068 7/1959 Rodda 3l395 X 2,908,840 10/1959 Anderson 313-95 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

1. A PHOTOTUBE COMPRISING A PHOTOELECTRIC CATHODE, A MASKING ELECTRODE PROVIDED WITH AN ELECTRON-RECEIVING APERTURE AND SPACED A PREDETERMINED DISTANCE FROM SAID CATHODE, ELECTRON LENS MEANS INTERPOSED BETWEEN SAID CATHODE AND A MASKING ELECTRODE FOR ACCLERATING AND FOCUSING THE ELECTRONS EMITTED BY SAID CATHODE IN AN ELECTRON IMAGE PLANE WHICH INCLUDES SAID APERTURE, SAID ELECTRONS BEING ACCELERATED TO A RELATIVELY HIGH ENERGY LEVEL BY SAID LENS MEAN BEFORE PASSING THROUGH SAID APERTURE, MEANS FOR COLLECTING THE HIGH ENERGY ELECTRONS WHICH PASS THROUGH SAID APERTURE, AND MEANS INCLUDING LOWER ENERGY ELECTRONS FROM PASSING THROUGH SAID APERTURE THEREBY PREVENTING AT LEAST A PORTION OF SUCH LOWER ENERGY ELECTRONS FROM REACHING SAID COLECTING MEANS. 